By HOPE YENWASHINGTON (AP) â¿¿ A record number of U.S. counties â¿¿ more than 1 in 3 â¿¿ are now dying off, hit by an aging population and weakened local economies that are spurring young adults to seek jobs and build families elsewhere. New 2012 census estimates released Thursday highlight the population shifts as the U.S. encounters its most sluggish growth levels since the Great Depression. The findings also reflect the increasing economic importance of foreign-born residents as the U.S. ponders an overhaul of a major 1965 federal immigration law. Without new immigrants, many metropolitan areas such as New York, Chicago, Detroit, Pittsburgh and St. Louis would have posted flat or negative population growth in the last year. "Immigrants are innovators, entrepreneurs, they're making things happen. They create jobs," said Michigan Gov. Rick Snyder, a Republican, at an immigration conference in his state last week. Saying Michigan should be a top destination for legal immigrants to come and boost Detroit and other struggling areas, Snyder made a special appeal: "Please come here." The growing attention on immigrants is coming mostly from areas of the Midwest and Northeast, which are seeing many of their residents leave after years of staying put during the downturn. With a slowly improving U.S. economy, young adults are now back on the move, departing traditional big cities to test the job market mostly in the South and West, which had sustained the biggest hits in the housing bust. Also seeing big declines now are rural and exurban areas, along with industrial sections of the Rust Belt. Census data show that 1,135 of the nation's 3,143 counties are now experiencing "natural decrease," where deaths exceed births. That's up from roughly 880 U.S. counties, or 1 in 4, in 2009. Already apparent in Japan and many European nations, natural decrease is now increasingly evident in large swaths of the U.S.